I am struggling mightily with the immune system. I've spent the entire last 2 days alternating between the textbook and the internet, and have watched darn near every youtube video out there...and I'm still not "getting it." I thought I was OK with the innate system, but now I'm not even sure about that. Are K cells and Cytotixic cells 2 different things? It seems like I keep seeing them both referred to as Killer Cells, but I thought Killer cells were in the innate system? I'm having a heck of a time with who needs a helper and who doesn't, and who presents antigens and who doesn't. One minute I see a difference in the systems and the next minute it seems like they're all doing the same thing. I have an exam next week and I'm trying so very hard to prepare. I've spent the whole weekend with this material and feel like I'm getting no where. If anyone has any words that might shed some light...I would be so very, very, greatful.

I think you might be confusing killer cells with natural killer cells (NK cells). Cytotoxic T-cells are the same as killer T-cells and are part of the induced immune system. NK cells are part of the innate system and circulate constantly.

The cytotoxic T lymphocytes are typically defined by their surface molecule CD8 (so called CD8+ T cells). They are close kin to the CD4+ helper T cells. Both of these cell types have to be presented antigens by antigen-presenting cells (APCs) in order to function.

For CD4+ cells, dendritic cells (DCs) are potent APCs and B lymphocytes (B cells) do so as well. So DCs and B cells gather antigens from their surroundings and display them to CD4+ cells, who then initiate immune response if the antigen is immunogenic (bacterial antigen, for example). DCs and B cells have so-called MHC II molecules on their surface and CD4+ cells only recognize antigens presented on MHC II.

CD8+ cells are mostly meant to fight intracellular parasites (usually viruses) and cancer cells, and thus recognize foreign antigens that are produced by the cells themselves. Thus, virtually any cell in the body is an APC for them. If a CD8+ cell recognizes a foreign antigen on a cell's surface, it kills this cells - hence the 'killer cell' name (this is to prevent viral infection or cancer from spreading). The "killing" happens usually by forcing the target cell undergo apoptosis. Almost all cells in the body have MHC I molecules on their surface and CD8+ cells recognize antigens only when they are presented on MHC I.

NK cells, to the contrary, do not require APCs and they do not adapt according to the type of antigen or infection (unlike T and B cells, who undergo clonal expansion and increased antibody affinity etc.). Thus, NK cells are considered to be a part of the innate immune system: they always recognize same, common patterns on microbial or cancer antigens.

There are many subtypes of all immune cells, including CD4+ and CD8+ cells as well as NK cells. Not all of the subtypes are even known yet. However, the key features are quite well known, and even though they might seem complex, there is at least neat logic behind all of it :)

There are two things: K and NK cells, the first one is a killing cell and the second one - a natural killing cell. And cytotoxiccells are recognized as TC cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system.

Kristine22 wrote:There are two things: K and NK cells, the first one is a killing cell and the second one - a natural killing cell. And cytotoxiccells are recognized as TC cells once they become activated and are generally classified as having a pre-defined cytotoxic role within the immune system.

+1 kristine, u have said right

you should understand the difference first between the killer and natural killer cells first.

T-lymphocytes are can be distinguished from NK (and B-cells) as it is having a special TCR (T-cell receptor)

Important functions:

T-cell: capable to recognize peptide antigen presented by MHC I. It is a part of adaptive immune system.

NK cells: this special type of immune system bridges adaptive immune system with the innate immune system. It is specialized in rejection of tumors and cells infected with viruses.

biohazard wrote:Hmmh, I'm quite certain I covered both the MHC I-dependet "killing cells" as well as NK cells in my previous post. But I guess little repetition does not hurt, so keep it coming guys :P

@biohazard- "lalajane" don't want an explanation man. I just made a clarification he needed. This is not called as repetition dude. :D ;) I just made it "cut-short"